Actually, that artwork was for the games, not the anime. So we don't know whether or not Litten will evolve.
It was. And Pikachu is avaliable in the games, like in every main game. They probably added Ash and Pikachu because they wanted to, but regardless, it's game concept art and I doubt they have anything planned for Litten's development that far into the future and the people who design the artwork . for the games aren't the same people who work on the show, so it isn't really any indication of how things will go.
Where I got the "Ash's Little might fully evolve" from is from this picture here.
Because while I don't have a source, a mod over at Serebii Forums who knows a lot about the anime said this. And whilst I don't like this person, in any sense of the word, I know there is no reason to lie about something like this.
There most likely is one, or it wouldn't have been said. Sometimes you don't need a physical source if a trustworthy person says so, but again, ask him or someone else on the forums. I don't go there too often, so I haven't see a lot of conversations, but regardless, it still seems like game concept art to me. Incineroar hasn't even appeared yet and concept art are very early ideas of a Pokemon will look like, which would mean it could have been drawn months and months before we first heard about Pokemon Sun and Moon, so if it's anime art, why sketch what it will look like for the anime, with it's only appearance so far in the future being in M20, which doesn't even feature a Popplio, Rowlet, or Mallow.
